Permselective membranes



Aprl 4, 1961 P. E. HOCH ET AL PERMSELECTIVE MEMBRANES Filed April 16, 1956 United States Patent O PERMSELECTIVE MEMBRANES Paul E. Hoch, Niagara Falls, and Paul Robitschek, lnifalo, N .Y., assignors to Hooker Chemical Corporation, Niagara Falls, N.Y;, a corporatio of New York Filed Apr. 16, 1956, Ser. No. 573,174

20 Claims. (CI. 204-301) This invention relates to new permselective membranes and to processes for producing them. More particularly this invention relates to new permselective membranes exhibiting chemical, electrical, electrochemical, ionexchange, permselective, physical, and mechanical properes of such nature as to render them suited for use in' ice the number of functional groups within the nembrane it hasbeen the practice of the prior art generally to produce what is called a heterogeneous memhrane, that is, one wherein clumps or heads of lon-exchange resin'are scattered throughout an inert resinous membrane. This tends to increase the strength of the membrane somewhat, but has had an unfavorable eect upon the other properties, since it is virtually impossible by this method to provide a sufiiciently uniform dissemination ofionexchange groups throughout the membrane. This method results in a membrane which has areas of high ionexchange group concentration surrounded by other areas containing no ion exchange groups whatever. As a result, when the membranes are immersed in the electrolytic solution, the high concentration areas absorb a large amount of water while the low concentration areas absorb none. This results in severe strains and stresses being produced within the membrane and eventually results in the membranes rupture and subsequent failure. Another i disadvantage is that the conductivity of a membrane of this type is unsatsfactorily low. Because of these limii tations the heterogeneous type of membrane has not enfabricated into the form of a sheet or pellicle. It was found that this sheet selectively allows certain ons to pass through while preventing the passage of othertypes of ions. -In effect, the sheet acts as an ionic sieve. This property is called permselectivity.

Many uses have been found for permselective membranes. One use is the purification of sea water by" removing the salts therefrom. Another use' is the concentration of dilute spent pickling acids, waste salts, and alkalis which result as by -products of Chemical processes. Still another use is the demneralzation of proteins. Another use is the separation of onssuch as amphoteric ones from non-amphoteric ones, and certain ons from others having different mobility or electron charges. Another use which has become most important' is the decompostion of ionic solutions by electrolysis where it is desired to maintain the decomposton products separate from one another. is the electrolysis of sodium chloride solution where itis desired to keep the sodium hydroxide which is produced Another exseparate fromthe reactant sodium chloride. ample is the production of substantially pure carbonates during electrolysis by introducing carbon dioxideinto caustic solutions which are maintained' substantially free of chlorideons. u t V I A number of 'permselective membranes for thepurposes set out above have been disclosed in the prior art. t Some workers have. [used "sulfonated ,phenolic resins;

Phenolic resins, howe'ver, have beentfound to be insuficient to withstand the rigors of electrolysis in the presence of caustic soda and chlorine., 5 e i Most of the attempts in the art tofp rovi de permselective membranes suitable for use in electrolysis have been made with ion-exchange resins in their commrcially available form. There the resn is generally inthe for'in of beads, and is so prepared that it has the maximum number of ion-exchange groups per unit weightof rlesin.'

This is necessary to give the resn a high ion-ex ch'ange Capacity. In the caseof a permselectivemembrane, however, it isnot necessary that the membraecontain the V greatest possible concentration of rionexchange groups.:

Only sufficient' groups areneededto provide the necessary conductivity and ionic transferg Coisequentlyhe concentration of the ion-exchange groups may he'somewhat reduced in order 'to provide increased structural strength. To improve the structural strength and to limit of the process, thus greatly joyed wide commercial acceptance. i With respect to thehomogeneous membranes, that is, membranes wheren the individual on-exchange groups are uniformly distributed throughout the membrane, the

i results ohtained in the art to date have here also proved i processes because of the inherent inahility of phenolic resins to withstand chemical attack. r In' addition, these, resins, according to their proponents, must be 'produced: With a certain amount of solvent present at all -stages increasing the difficulties and cost of production. t ti It is an object of this invention to provide new cation permselective membranes. It is also an object to provide u such membranes which have good'conductivity. Itis a further object of this nvention'to provide suchtmemt-l branes which have excellent mechanical properties ren-x An important examplei throughout the mem industrial applicationsfmay the producedhy .copolymercarbon-to-carbon double'bonds; *and (D)'{`a polymerizaf tion catalyst. The 'copol'ymer thus formed inthe'shap `In' accordance :with this invention'the membrane 's: he

v drolysiscatalystsuch 'as dering 'them suitable for industrial operations,j especially@ bf ,where the membrane is subject to attack by Corrosive ree; actants and products' It is still a further object of this i invention to provide such permselective membranes'whclr` will contain the i en-exchange groups uniformly distributed" hrane and chemically comhinedwith the resn molecules. p It is also an object of thisinvention 5 fto provide a permselective membranewhich may he fah i rcated substantially free from any liquid -solvent ;andi maintained so 'until just prior to its use in a cell(` Stillg further'objeets will becomeapparent to those skilled in the art on further consideration of-the disclosure made 5 hereinaften t J ,V x It has now been found that homogeneous permselective membranes 'containing the desired propertiesifforusein izing a mixture comprising (A) `a composition'fselected. p

from-the group consisting of (1) an olefinic mono'ner 1 i containing a carboxyl group, a nd (2) a mixture oran. i olefinic monomer containing a group'which may he`su h-` sequently hydrolyzed to form a carboxylc acid ?group? monomer free from 'potentially active ion-xhange groups; (C) an olefinic monomer containing. at 'least two of a` pellicle or'membrane'is substantia'lly free from wate treated, in an aqueous solution "intthe presence a 'h sodium hydroxide to co'nv rt &978,402

potentially active functional groups to carboxyl groups or salts thereof, and to introduce Water into the membrane. It has been found that membranes produced in this manner have inherently good physical properties, high eiciency in selectively transferring ions, and good conduc tivi ?s used in the paragraph above' and throughout the specication and claims the terms have the following meanings. Carboxyl groups" include the groups in their acid form containing hydrogen, in their anhydride form, and in their salt form where the hydrogen is replaced by a metal. The term carboxylic compound" is limited to the group conssting of carboxylic acids, carboxylic anhydrides, carboxylic acid chlorides and carboxylic salts. The term group which may be subsequently hydrolyzed to form a carboxyl group" comprises such groups as esters and amides of a carboxylic acid, and nitrile groups, all of which upon treatment with the proper hydrolyzing medium may be converted into carboxylic acids or salts thereof. The term monomer free from a potentially active ich-exchange group means that there is no group attached to the monomer which upon treatment in any manner such as by hydrolysis will form a group which would act as an on exchange group. The term "potentially active ionexchange groups" include esters, antides, or acid chlorides of carboxylic acids, nitrile groups, carboxylic acid and anhydride groups, or any other group which may be treated such as by hydrolysis to form carboxyl ion-exchange groups or having ion exchange properties. Free from liquid solvents" means free from solvents such as water, toluene, benzene, diethylbenzene, alcohols such as isopropanol, ketones such as cyclohexanone, ethers such as dioxane, halogenated hydrocarbons such as ethylene chloride, hydrocarbons such as heptane and in general, any liquid which will dissolve in the resinous mixture but will not copolymerize therewith. Also, as used in the specification and claims, the term membrane includes "sheet" or "pellicle' Reference is made to the figure which is given to illustrate a preferred use of our invention in the electrolysis of Chemical compounds, but is not to be construed as limiting, except as defined in the appended claims. A container 1 is divided by the permselective diaphragm'Z of our invention into an anode compartment containing an anode 3 in contact with anolyte 4, and a cathode compartment containing a cathode 5 in contact with catholyte 6.

The membrane formed by the present invention comprises an addition copolymer of several monomers, with the active ion-exchange groups homogeneously distributed throughout the membrane in such a manner that the strength of the membrane is not adversely aected thereby.` The potentially active functional groups of the ioneXchange-active monomer may be either carboxyl groups or groups which may be hydrolyzed subsequent to the polymerization to produce carboxyl groups. In every case however, the ultimate active ich-exchange group will be a carboxyl group. Where the ion-exchange active monomer as Originally introduced already contains the carboxyl group attached thereto, subsequent hydrolysis is of course, then unnecessary and the membranes need only to be treated to introduce water and the desired catons.

The permselective membranes of this invention, areprepared generally by mixing together the desired monomers with a polymerization Catalyst, and maintaining the 'mixture at the necessary polymerization temperature in the absence of any liquid solvent until a hard sheet-tikecopolymer forms. The monomeric mixture is made up of several Components. The first is an olefinc polymeriza ble monomer which either contains a carboxyl group attached thereto or else contains a group which may subsequently be hydrolyzed to produce a carboxyl group' The second Component is an olefinic polymerizabl'e monomer which contains neither a carboxyl group; nor any group which may be hydrolyzed to form a carboxyl group,

nor any other ion-exchange functional group. This component acts as a diluent. The third Component consists of a difunctional olenic group-containing-monomer which will cross-link the polymer chans of the composition when cured together in the presence of a polymerization catalyst. The cured membranes thus formed by the preceding process are solid sheet-like membranes which are substantially free from any liquid solvent. in order to introduce water within the structure and to hydrolyze the carboxyl-forming groups, the membrane is then treated in a hydrolyzing medium such as aqueous sodiun hydroxide. This succeeds in converting the carboxylforming groups into functional ion-exchange carboxyl groups, and also succeeds in introducing water of hydration, rendering the membrane electrically conductive. The resulting membrane is now capable of selectively passing cations while at the same time repelling anions. It now constitutes a copolymer to which there are attached at uniform intervals throughout the membrane functional on-exchange groups which are either carboxyi groups orsalts thereof.

The thickness of the membranes made by this invcntion is not critical, but will depend upon the dcsired use. In general, suitable membranes may be from :s little as about one-sixteenth inch to as much as about one inch thick. A thick membrane will have a longer useful life. However, the resistance of the membrane increases proportionally to its thickness and if the membrane is made increasingly thicker, a value will be attained above which the resistance is too great for practical use.

The functional monomer should be an olefinic compound selected from the group consisting of carboxylic acids, carboxylc anhydrides, carboxylic chlorides, carboxylic esters, carboxylic amides, nitriles, and mixtures thereof. Among the carboxylic acids and anhydridcs which may be used are acrylic and methacrylic acid, maleic acid and anhydride, fumaric acid, itaconic acid and anhydride, cinnamic acid and anhydride, vinyl benzoic acid and its anhydride, crotonic and isocrotonic acid and their anhydrides, butanoic acid, luconic acid, alpha hydrocinnamic acid, aconitic acid, chloromaleic acid and its anhydride, ethyl maleic acid and its anhydride etc. Among the esters are methyl acrylate, ethyl acrylate, Z-ethyl-hexyl acrylate, the esters of methacrylic acid, the esters and half esters of maleic and fumaric acid, such as methyl and ethyl maleates, and ethyl and methyl fumarates. Unsuturated acids such as acetylene dicarboxylic acid which contain triple bonds may also be used. in addition, the amides and acid chlorides of the above named acids and anhydrides may also be used. Among the nitriles, acrylonitrile may be used.

The proportion of the` ion-exchange-group-containing monomer used determines the io`n-exchange functional group capacity and consequently must be kept within well defined limits. A The ion-exchange functional group capacity, such as when measured by the millequivalents of lon-exchange radical per gram of dry resin, is an important factor in the determination of both the strength properties of the membrane and its electrical conductivity. The higher the functional group Capacity, the greater will be the watercontent and swelling pressure of the mem-, bran'e during and after hydrolysis whe'n measured at any given external electrolytic concentration. For instance when the milliequivalent Capacity per gram is over 6.0 the polymerized and hydrolyzed membrane has generaliy lower` structural strength. As'this value of milliequivalent Capacity per gram' becomes lower the hyd'olyzel membrane becomes more rigid. The range of functionai group capacity should be between about 1.0 and 6 n-.ii'ziequivalents ofion-exchange radicals per gram of dry poly mexized rein. The preferred range is about 2.5 and` 3.0 m illiequivalents. i

The diluent component, for` example styrenc, hasan important function: in` the permselective` membrance.

First, it serves `to decrease the ina-exchange functional i strength or rigidity of the final membrane.

where the amount of inert monomer is increased to greater r cent.

y polymer. i

group concentration and thereby decrease the swelling stress produced within the membrane at the time it is hydrolyzed and during the time it is Operating in contact with the ionic solution. Additionally, as the diluent content increases, the structural rigidity, inertness, and strength of the membrane also increases. For instance, a membrane containing a 27 mole percent styrene content has a 126 percent Volume change upon hydrolysis. A membrane containing a 64 percent styrene content has only a 12 percent Volume change upon hydrolysis. This latter sheet is much more rigid and easier to handle. The only limiting consideration of the inert diluent concentr-ation is that, as the diluent content is increased, the couductivity of the membrane decreases proportionally. Thus, the diluent content cannot beincreased beyond the point where the conductivity goes below the useful level. The preferred range of the diluent is between about 50 and 85 mole percent, although a range of 25 percent to 90 percentmay be used in some cases. When it becomes, muchless than 50 the, strength of the membrane begins to fall ofi. When it is greater-than 85 mole percent the resistance of the membrane becomes too high as there is then not a sufficently high concentration of ion-exchange groups. i

The diluent may comprise one or more of the following: styrene, vinyl naphthalene, ethyl vinyl-ether, isobutyl vinyl ether, methyl styrenes such as alpha methyl styrene, butadiene, isoprene, chloroprene, chlorostyrenes, fiuorostyrenes, chlorinated ethylenes, vinyl chloride, vinylidene chloride, etc; Fluorine-containing olefinic monomers such as' trichlorofiuoroethylene, -dichlorodifluoroethylene, and tetrauororethylene may be used if low temperature or high pressure polymerization conditions are maintained.

It is desirable in most cases to cross-link the polymers of the invention to form an insoluble infusible mem brane. This is accomplished by introducing into the polym-erizable mixture a polyfunctional cross-linking agent, containing at least two olefinic double bonds such a's divinylbeuzene. The cross-linking has a pronounced eifect upon the rigidity of the polymer and thus' aids in' controlling the swelling stress during hydrolysis and increasing the physical strength of the membrane. It has hen observed that 4 mole percent content of divinylbenzene gives sheets which are more brittle than those containing 2 percent or less.` The effect of the crosslinking agent is closely interlocked with the relative amount of the inert monomer. As the amount of inert monomer is increased, the amount of cross-linking agent may be decreased without any adverse effect upon the In t fact,

than 65 percent, satisfaotory membranes may be produced even in the absenceof any 'cross-linking agent.

. The preferred proportion of the cross-'linking agent is between about 0.25 and 4.0 mole percent, although a range of 0.1 to 8.0 mole percentmay be used. Other cro'sslinking monomers that may be used are divinyl ether;

butadene, and other aliphatic diolefins or polyolefins. p The polymerization catalyst may be any of a large number of common addition polymerization catalysts.

The preferred catalysts are free radical polymerization catalysts such as benzoyl peroxide, Organic 'hydroperoides, such as "cumyl hydroperoxide, and peroxides such .as dicumylperoxide. In some' cases the ionic polymerization-catalyst may be used such as boron trichloride etherate, alnninum-ohloride, stannic chlon'de, ,and others. The catalysts may be used in any amounts generally dis closed in the prior art. As little as one-tenth of one percent based on the weight of the copolyrner may "he employed successfully. There is no critcalupperlimit. However, little may be gained by exceeding three er The preferred range i's from about. one toabout one and one-half percent by' weight based on the total .Themembranes ofthe present invention ,may be fabricated from the mixture of monomers in any onejoflf many techniques which are known to the art. Onemeth-j e od is to cast the liquid mixture in a mold and heat, the mold until the polymerization is complete. This *is very: `Conveniently donebetween two glass-plates. One prob: lem which presents itself in using the casting technique is that an appreciable 'shriuking or reducton in volume' occurs during the curing process which in some cases may reach percent or more of the original Volume. If a rigid mold is used, imperfections such as voids, fissures and cracks may result during the casting. One method that has proved successful for avoiding this problem is to separate the glass plates forming the sides of the mold with a flexible gasket material such as rubber tubing. The mold may then be placed ina bolted frame and V coil sprngs placed over each bolt to exert the 'force upon the mold sides, teuding to make them follow the dimensions of the curing polymer.

introduces water of hydration into the men brane allows the membrane to become electrically conducting.`

, Second, it hydrolyzes the potentially active functional groups such as the carboxylic esters, amides, chloricles',` and the nitrile groups to carboxyl groups or salts thereof. This renders these groups capable of acting as ion-exchange groups. It is generally preferred that the hydrosisting of carboxylic acids, carboxylie auhydrides, and mio monomer containing agroup which may be hydrolyz edf merization. This material should preferablybean olecitraconic' anhydride, aconitic 'anhydride, itaco'nic anhyf dride,'etc. ,The amount of this material added should:`

'very effective in resistngthe Corrosive' action coft-the 7 'severe conditions encountered in a commercial l electrq-' u lytic application. In this respect it is far superior to'thelyzing medium be an aqueous solution of a strong base such'as sodium or potassium hydroxide. In 'some cases, however, strong acd solutions may be used, although they may necessitate a longer treatment The concert: trations of the hy drolyzing media are not critical but should generally be at least 5 percent by weight of the alkali or acd based on the weight of the solution. The time of treatment varies depending upon the membrane composition. It may vary from several hours to as much as several days. The preferred temperature sfromabout degrees to. about degrees centigrade, al though lower or higher temperaturcs may be used. Sub 1 sequent to the hydrolysis treatment themembrane may I be placed in the cell in which it is to function( However, in 'order to condition it, the membrane may, subsequen-t to hydrolysis and prior to use in a cell, be treated in asolution which contains the same ions at approxi mately the same activities as those ofthe solutionin the electrolytic cell in which ;the membrane is subsequently 7 to function e p p A y p "It has been found that in order to facilitate the hy-r drolysis process of a membrane which has been preparedf free from any liquid solvent according to this invention p and where the functional component is in the form of' i to form a carboxyl group, it is necessary 'to admianf i v i' unsaturated organicacid or anhydride prior to the poly finic compound andmay' be selected from the .group con' tures thereof Among the compounds which are s uited for this purpose'` are acrylic acid, maleic anhydride,

be at least about one percent -by weight 'based oi'the total polymer.` u e i The permselective membranes of this nvention have manyadvantages over those of'the `prior art. "First, the. copolymer formed is of such a composition that it is extremely reactive products used andproduced and the u traditional phenolic resin ion-exchange membranes '-jSeco d, because the v mh spread uniformly. throughout the 'membrane'a d a'enot concentrated in localized 'areas which are in turn surrounded by inert polymer areas, as in the case of the membranes composed of lon-exchange beads Suspended in inert polymeric sheets, 'the membranes are physically stronger and less subject to cracking when in contact with the electrolyzing solution. During the hydrolysis and during electrolysis the ieri-exchange groups become highly hydrated. When the ion-exchange groups are uniformly distributed throughout the membrane, the 1nternal stress produced is not unduly severe. However, when a high ion-exchange group concentration area such as an ion-exchange head becomes hydrated, severe stress is produced and the membrane becomes highly susceptible to cracking.

Third, because the membranes of the present inventon are polymerized free from any liquid solvent, they are more readily polymerized on a commercial scale, they form stronger and more resistant membranes, and can be more easily handled and stored until ready for use in a cell.

Pourth, because in the membranes of the present invention the ich-exchange groups are uniformly distributed and are not separated by large masses of inert polyners, as in the case of the membranes made from ion-exchange heads, the conductivities are higher than in the case of the heterogeneous membranes of the prior art. Table I below gives a comparison of the conductivities of the mernbranes of the present invention with those of some heterogeneous membranes of the art.

For all the conductivity measurements of the above, a ,5 M sodium chloride solution was used. Amberplex C-l isa membrane composed of ich-exchange heads which are the polymerzation product of sulfonated styrene and divinylbenzene and which are subsequently imbedded in a polyethylene sheet, the proportions being 70-75 parts polyethylene; 20-25 percent sulfonated polystyrene and 6-8 percent divinylbenzene. Perrnutit cationic diaphragm No. 747 is a mixture of 25 grams methacrylic acid and one gram divinylbenzene, also imbedded in an inert sheet of polyethylene. Since the membrane of the present invention is more conductive than both of those shown above, they may be used in a thicker form, as a result of which they will have a much longer useful life.

The following exanples will serve to illustrate the present invention and ?the improvements resulting therefrom. i

Example 1 A casting mold was prepared by separating two pieces of plate glass by a tubular piece of rubber used as a gasket. The glass mold was then placed in a .rectangular wooden reinforced f'ame. the outer edges of the Wood tran-ie. Each bolt was fitted with a col type spring, washers, and wing `nuts. The springs served to keep pressure on the casting during polymerization. Two hypodermic needles were inserted in one corner of the glass ,mold between the glass and the rubber gasket, permitting the filling of the mold with the casting material and all'owing ari escape vent for the displaced` air.

Bolts were placed through' Example 2 A large Erlenmeyer fiask was charged with one thousand grams of styrene, 3165 grams of ethyl acrylate, 150 grarns of maleic anhydride, 54.7 grams of divinylbenzene-ethyl styrene solution containing 56.2 percent divinylbenzene, and 15 grams of benzoyl peroxide. The mixture was stirred until all solids dissolved. Then the solution was filtered. This monomeric solution was then placed in a separatory funnel in an elevated position. A piece of plastic tubing was attached to the funnel and to one of the needles of the mold prepared above in Example 1. The solution was then allowed to flow into the casting mold. When the mold was filled, the needles were removed and the mold placed in a vertical position to test for leaks. It was then lowered into a chest lined with steam-heated copper coils which had been preheated to 72 to 75 degrees centigrade. Polymerization took place within one hour, but the filled mold was allowed to remain in the cabinet for a total of 18 hours. It was then cooled to room temperature, removed from the chest, and the glass plates separated from -the casting. The resulting casting was a rigid polymeric sheet, which was trimmed free of the gasket. The sheet was now rigid, had good strength and was substantially free from any liquid solvent.

Example 3 The sheet prepared above in Example 2 was treated to convet the ester-substituted carboxylates to sodium salts of carboxylic acids, and in addition to introduce water into the membrane so that the cations and their charge could be transported through the membrane. For this purpose the sheet produced above was placed in atank containing 17 percent sodium hyd'oxide which had been heated to a temperature of to degrees centigrade. The? sheet was then left in the tank until the dimensions no longer changed, the completion of this step requiring about 3 to 4 days. During this tirne the sheet had in c'eased approximately 41 percent in length and width. It was quite pliable, translucent, and tough. It had a' high tensile strength. As thus prepared, the sheet was ready for use as a cation-exchange diaphragm for the purpose of electrolyzing a salt solution.

Example 4 e The membrane produced above in Example 3 was placed in a two-compartment test cell in order to test its usefulness in producing chlorine and caustic soda. The test cell comprised a vessel containing two compartments which were separated from each other by the membrane produced above. Sodium chlorine brine was fed continuously at approximately 25.5 percent sodium chloride and was depleted to approximately 22 to 23 percent sodium chloride. Water was added contnuously to the cathode compartment to maintain" a concentration of approximately 25 to 40 g'arns per liter of sodium hydroxide. The anode consisted of a graphte block, and the cathode of a steel screen. The membrane was spaccd from the anode by a one-fourth inch thick soft rubber gasket. The cathode screen was placed in contact with the diaphragm and spaced one-half inch from the steel back plate of the cell. The active area of the diaphragm was 7.9 square inches. The svoltage was adjusted to pass a S ampere current through the cell, or a current density of 91 amperes per square foot of membraue. Chlorine was continuously produced at the anode, hydrogen at the cathode, and substantally pure sodium hydroxide in the cathode conpartment. The membrane exhibited high v currentefi iciency and a long useful life.

The following examples 'demonstrate the methods by which thevarious properties of the membranes of the present invention were determined. i

` The following 'is the calculation used:

Example 5 with this process requiring from one to four days. Dur

ing this period the level of the lquid in the beaker was maintained approximately at the starting level by the adcatholytc exhaust for high hloride content with silver' diton of distilled water. At the end of the heating period the aqueous solution was carefully poured into a 1 liter volumetric flask using distilled water to wash in all traces of the caustic solution. The hydrolyzed membrane was then washed successvely with 50 cc. of distilled water, 250 cc. portions of 10 percent brine, and again with 50 cc. of distilled Water. The washing was done by swirling the wash lquid around the membrane in the bottom of the beaker for a ten minute period. Each successive wash was added to the volumetrc ftask in the manner described above. After the transfer of all the washes the Volume of the lquid in the volumetric flask was adjusted to the mark with distilled water, the flask Shaken to insure thorough mixing and a 25 cc. sample of the resulting solution tritrated with 0.1 N sulfuric acid using methyl orange as the indicator. The total amount of caustic utilized during the hydrolysis was then calculated and this amount related to the number of equivalents of hydrolyzable groups available in a membrane.

` Example '6 .compartment, and water was added to the cathode compartment at a rate sufficient to permit the generation of about 3 percent caustic soda, The total amount of charge was measured by an ampere-hour meter. The catholyte overflow liquor was collected and the amount of caustic produced within the period 'of time was analyzed, The i theoretical amount of equivalents of caustic soda Was determned* from calculation of the amount of electrical charge which had passed during the measurementperiod; The current elficiency was then obtained'by dividingjthe' equivalents of caustic soda analyzed by the amount that should theoretically have been formed by the 'amount of electricity passed and multiplying this quotient by "100,

3,6 p p a equivr' eaustie 4 EEquiv. caustic'by analysis X OO f Equiv; caustic by tli eory v Example 7 i The membrane was ;placed in atWOr-COmpartment, 5 am:

to flowrinto the anode compartment and water; into the,

cathode compartmentqlhe water; flowwas adjusted to generate B'-prcerit caustie 'in the catholyte, j liailures in; thelnenibrane were detected byperiodicallytesting' th z v lata' 4",i image t,

theory nitrate solution.

Example 8 The permselectivity oftheion-exchange membranes was determned as follows:

Permselectivity may be defined mathematically as -follows:

mobile ionmobile ion i permselectvty Where r Mobile ion=transference number of the mobile ion in mobile ion= E=measured voltage Where:

t =transference number of the positive ion t =transference number *of the negative ion :gas constant r i T=absolute temperature salt concentration of the other. The concentrations weref 4 `-on`the order'of 0.1 Net-0.01 N in most of the detern'ina` u .eter. v

V transference numbers u; t were readily. available 'n Equatio n tz labove was `readily calculated. r Then'from 4 Equation .1 the determined i A g; t The examples listed iiT able II belowtcontan.data for ;The membraneajlife testwas performed as follows:

70. pere cell as described in Example 4 Brine was allowed i partnent cell -describedein Example 4;aboye,--'wa" use were testdaccording "to t x fi' 19'.

The procedure for determination of ermselectivity i i was as follows: The wet preconditoned membrane was i i clamped in a two-compartment cell. Into each compartment were continuously passed `dlutesodiurn chloride solutions. Thejconcentration of the`solution`s -wa s'-ac-` curately known'and the one soluton had about twice the tion s. The potential developed across the membranesby v i this cell (E) was measured (using silver-silver chloride standard electrodes, one immersed iii-each'corrp'artmentV V in series with the membranes) by a sensitive potentiom- The value of E in the Equation above' wasr'eadil y calulated from the known value of 'the salt concentration v' (fromwhich the activities zfia 'were easily calculated' using literature values for ;the activitycoefiicient) The literature from the known' activities o ffthe ions.

With E and E known the value of mobile ;ion in;

permselectivity of the' membrane Masi membrane compositionand performancesin a 'test 'ce ofa number of the' various pecie s, of-the present nven ton. I-n every case below the one membrare two-com for testing' purposes.: :Th'e' properties -of ,the' mernbrane the methods decrib d y ber tubing gaskct.

'TABLE II Monomer, Gratis I Current t Permse- E Lite in Example Number I tt r lectlvity, ciency, Days Diluting Functlonal Monomer Snsltlzlng Monomer Cros`s`=Llnking` Percent Percent Monomer Mouomer Example 9 Styrene, 66.5 Ethyl Acrylate, 22; Maleic Anhydrld, 9.8... Dlsvilnylben zteu 100 94.0 36

, i o u ion, .o.

Example 10 Styrene, 132.4 Mcthyl Acrylate. '41 Maleic Anhydrlde, 19.4.. Di vilnlbenzee 97. 9

. to u lOIl, 7.

Example 11 Styreue, 16l Methyl Mcthacrylate, 29.3 Methacrylic Acid, 28.8 Disvilnlbenee B 79.0 o u ion,

Example 12 Styreu, l57. Diethyltumarate, 63.2..." Maleic Anhydrlde, 2l.1 Dlsvinybeneo 82. 7 74.0 25

t e o u on,

Example 13 Styreue, 183.5. Acrylonitrile, 29.8 Maleic Anhydride, 26.2- Dlsvilnlhenere e o u on,

Example 14 Styrene, 163. Ethylacrylate, 50.3 Cit'acouie Anhydrlde, Divlnylbenzene 83,(5 69.0 i

26:'1 Solution, 9.8.

Example 15 Styreue, 90.8 Itaconic Anhydride, 1L2.- Divinylbenzene 89 80.0

Solution, 5.6.

Example 16 Styrene, 159..-- Acrylic acid, 77.5; i Divinylbenzcne 80 03.0

i V Solution, 14.

Example 17 Styrene, 50 z-etlylhexylacrylate, 8.55; Maleic Anhydrlde, 4.7. Divlnylbenzene 95 93.0 39

Ethylacrylate, 34. Solution, 4.7.

The divinylbenzene 'solution was made up of 52 percent divinylbenzene and 48 percent ethyl styrene. One percent benzoyl peroxide was used as a Catalyst in every case. Where the values are left blank, the corresponding measurement was not made.

The following example der'onstrates a membrane which contains no cross-.linking agent.

Example 18 A large Erlenmeyer flask was charged with 900 grams styrene and 100 grams maleic anhydride and 10 grams of henzoyl peroxide. The mixture was then placed in .a casting mold and polymcrized according to the method of Example 2. The solid sheet was then treatcd according to the method described in Example 3, in order to introduee waterof hydration. This membrane exhibited a permselectivity of 94.9 percent. When tested in a single membrane cell for the production of chlorine and caustic soda as described above, the membrane had a useful life of 42 days.

The following examples illustrate the production and testing of permselective membranes included within the present nvention in a threo-compartment cell containing two caton permselective membranes for the simultaneous production of potassium hydroxide and potassium carbonate.

Example 19 A solution of 132 grarns styrene, 41 grams rnethyl methacrylate, 19.4 grams maleic anhydride, 7.8 gr'ams of divinylbenzene ethyl styrene solution 'containing 52 percent divnylbenzene, and 2 grams of bnzoyl peroxde were mixed toeflfect solution, and were poured into a casting cell. This cellconsisted of two rcctan'gular sheet's of one-fourth inch plate glass separatcd by -a flexible' rub-I The glass cell was contained in a wooden frame constructed in such a manner that pres sure could be contiiuou'sly applied to the exterior of the glass cell by bolts and s'prings or rubber tubin'g. The `casting cell was then placed in an oven'hcated to 62 degres centigrade and held there for 16 hours: Atthis point the temperature was raised to 95-100 'degrees centi'grade and held there for three hours. The i'igid polyrneric 'sheet was then removed. 4

Peces of this sheet were cut'andrplaced 'in `90 9-5 :de-' grees ccntigrade l'3 percent' potassium hydfoxi'de for one to two weeks. The rlgd but more plable n'em'brahc now r scribed above.

for the production oi' potassium hydroxide and potassium carbonate. This cell eomprised a container, an anode compartrnent which contained a graphite anodc and which was separated from the center compartment by a cationic permselective diaphragrn of this invention, and a cathode compartnent which contained a steel cathode and which was separated from a center compartment by a similar cationic p'ermselective membrane. This cell is disclosed in copending application for Method and Apparatus for Electrolysis, Serial No. 327,l82, filed December 22, 1952, by Sidney G. Osborne and George T. Miller, now abandoned. The anode compartment contained a 25 percent potassium chlo'ride solution which was separated from a center compartment by the permsclective membrane de- The center compartment coutained 20 percent potassium carbonate. The cathode compartment contained a 40 percent potassium hydroxide solution. When the current was applied through the electrodes of the cell, chloride ions were discharged at the anode with the production of chlorine, while the potassium ions were attracted toward the cathode with the production of hydrogen and potassium hydroxide. The cationic ion exchange membranes ,acted as a negatively charged screen and allowed the passage of the positively charged potassium ion into the cathode compartment by a series of transfers along the carboxylic acid groups incorporated in the membrane. The reverse passage of negative ions from the anodc or cathode compartments was prevented by the negatively charged carboxylic acid groups which tended to repel the anionsl Hydrogen ions were discharged at the cathode with the production of hydrogcn gas while at the same time the migration of the potassium ions into the cathode compartmcnt resulted in the producmoved as an end product. The coucentration of the potascontaining approxim'atelv32 40-percent`water was capai ble of'conduc'ting current in an electrolytic solution 'in an ion-sclective marinar. It indicated a rper'rnselc ctivity of 97.9 percent in a two-conipa'rtmcrit concentration cell `using electrolyteof appi'oximately 0.1i'and"0.01 Nin each compartrnent. e

i i :ExampIIe 'ZO The membrane produced as in er'nt atve was placed in a three compartment electrolytic; cell and'used` i t per square foot'was maintained with respect' to the dia# phragns requiring avoltag' `ofabout 4.6 volts. Atthe end 'of the process very'pure potassiurn hydroxide and :potassiumtarbonate were obtained in excellent yield.

'* The ecnoniical operation of this 'cell is dependent upon 'the elficih'cyf *the two -ioi `*exchange membranes in separatiug the cell intothrce compartments. If these ions' sievesi'- are operating ,at high' efficiency, hydroxyl ions are prevented from migrating into the anode compartment under the influence of the electric field. This results in a high cell current efliciency and in the production of chlorine which is freefrom oxygen. Another function of the permselective membranes is to prevent the diffusion of chloride ions from the anode compartment into the center and cathode compartments. Prevention of this ditfusion results in the production of very pure grades of potassium carbonate and potassium hydroxide, both of which are subs'tantially free from chloride ions.

The cation permselective diaphragms of the present invention may also be used to electrolyze Organic salts' 346,365, filed April 2, 1953, now Patent No. 2,967,806,

by Sidney G. Osborne and George T. Miller. Here two cation permselective membranes separate a center com partment from both the anode compartment and the The salt such -as sodium formate or sodium oxylate is placed in a center compartment cathode compartment.

and upon the application of electrical current, the cations migrate to the cathode compartment and the acd remains in the center compartment. i It has been additionally found that the principlesof the present invention may be utilized to prepare novel anion permselective membranes. polymeric type diaphragm which is composed of a homogeneous copolymer which contains attached .thereto and uniformly distributed throughout anion exchange groups rendering the membrane anionically permselective. The following example describes this preparation. i

Example 21 A solution of 31.2 grams styrene, 19.1 grams 4-vinyl pyridine, 2.3 grams of divnylbenzene-ethyl styrene solution, and 0.05 gram of benzoyl peroxide was prepared in an Erlenmeyer flask. This solution was placed in a' casting mold described above. The mold containing the solution was placed in an oven and maintained at 75` degrees centigrade f or 19 hours. A clear, rigid, ambel'- colored plastic sheet was ob'tained.

This sheet was then placed in 20 percent hot sulfuric acid solution maintained at 90 to 95 degrees centigrade. This treatment' continued until the sheet had swelled about 80 percent in Volume. In 'this form it wa's electrically conductive, had a permselectivty greater than 85 percent, and operated satisfactorily as an anion permiselective membrane. L i r V- The anion permselective membrane produced above in Example 21 may be used in cells either alone or in conjunction with the cation permselective membrane of this invention for a large number of uses. One such use is described in copending application Serial No. 267,846, filed January 23, 1952, by Sidney G. Osborne and George T. Miller, which discloses the electrolysis of sodium sulfate solution in'a three-compartment electrolytic cell. In this cell the salt is introduced into the center compartment which is separated from the cathode compartment by a cation permselective membrane, and which is separated from the anode compartment by an anion permselective membrane. A graphite anode and a steel cathode are inserted and a current applied to the electrodes. The operation of this cell results in the production of sulfuric acid. Another use of this membrane in conjunction with the cation permselective membranes of this invention is in the de-ionization of seat water. The method for so doing is widely described in literature and particularly in United States Patent No. 2,694,680 which discloses a method of alternately using a plurality of anion and catonpermselective membranes to transfer electrolytes from one solution to another and thus efect a removal of ions from the material which it is desired to purify. -As stated therein, this method This results in a copermselectivemembrane which consists essentially of polymerizing amixture consisting essentially of (I) an addition polymerization catalyst, and (II) a mixture of monomers capable of addition copolymerization consisting essentially of '(A) an ion-exchange-active material selected from the group consisting of (1) a polymerizable olefinic monomer containing a carboxylic group, (2) a polymerizable olefinic monomer containing a group which may be subsequently hydrolyzed to forma carboxylic group, (3) mixtures of (1) and (2), and (4) a polymerizable olefinic monomer containing an anion-' exchange group, and (B) a polymerizable ion-exchangeinactive monomeric diluent selected from the group consisting of (a) a polymerzable olefinic monomer, (b)` a cross-linking olefinic monomer containing at least two' polymerizable carbon-to-carbon double bonds in an amount not more than 8.0 mol percent of the copolymeric composition, and (c) mixtures of (a) and (b); producing a dry homogeneous polymerized membrane; and subsequently converting the potentially active functional groups to ion-exchange groups and introducing water of hydration into the membrane by hydrolyzing in an aqueous solution containing a hydrolysis catalyst to the extent necessary to produce ich-exchange groups uniformly distributed throughout said membrane in an amount from about one to about six milliequivalents per gram of dry membrane.

2. A permselective membrane formed in accordance' of (I) `an addition polymerization catalyst and (II) a r mixture of polymerized monomers capable of addition copolymerization consisting essentially of (A) a polymerizable olefinic monomer containing an anion-exchange group; (B) a polymerizable ion-exchange-inactve olefinic monomer; and (C) a cross-linking olefinic mono'- mer containing at least two polymerizable carbon-tocarbon double bonds, said cross-linking olefinic monomer being present in an amount not more than 8.0 mol percerit of the dry copolymeric composition. 4 i V 4. A process for the production of a dry solid resinous i cationic permselective membrane which 'consists essen tially of polymerizing a mixture consisting essentially;

of (I) an addition polymeriation Catalyst and (I-I)' a mixture of monomers capable of addition copolymeri zation consisting essentially of (A) an ion-exchangeactive material selected from the group consisting' of (1) a polymerizable olefinic monomer containing a cart boxylic group and (2) a polymerizable olefinic monomer containing a radical which may be hydrolyzed to form a carboxyl group and which additionalyl'contains' in admixture a polymerizable olefinic carboxylic compound, (B) a polymerizable ion-exchange-inactive olefinic monomer, and (C) a cross-linking olefinic mono mer containing at least two polymerizable carbon-tocarbon double bonds; producing a, dry homogeneous v polymerized membrane; andsubsequently convertng the potentially active functional groups to carboxyl groups and introducing water of hydration into the membrane i by hydrolyzing it in an aqueous solution containing a V hydrolysis'catalyst to the extent necessary to produce carboxyl groups uniformly distributed throughout said' r membrane in an amount from about one to about six milliequivalents per gram of dry membrane.

5. A process according to claim 4 whereinsaid olefinic J V,

monomer containing a group which may be subsequently hydrolyzed to form a carboxyl group is a carboxylic ester.

6. A process according to claim 4 wherein said olefinic monomer containing a group which may be subsequently hydrolyzed to form a carboxyl group is an ester of acry1ic acid.

7. A process according to claim 4 wherein said olefinic monomer containing a carboxylic group is acrylic acid.

8. A process according to claim 4 wherein said olefinic monomer containing a group which may be subsequently hydrolyzed to form a carboxyl group is a carboxylic amide.

9. A process according to claim 4 wherein said olefinic monomer containing a group which may be subsequently hydrolyzed to form a carboxyl group is a nitrile.

10. A permselective membrane formed in accordance with the process of claim 4.

11. A homogeneous solid resinous permselective membrane according to claim wherein said olefinic monomer containing a carboxylc group is acrylic acid.

12. A homogeneous solid resinous permselective membrane according to claim 10 wherein said olefinic monomer containing a carboxylic group is maleic anhydrde.

13. A homogeneous solid resinous permselective membrane according to claim 10 wherein the polymerization product is a mixture consisting essentially of (A) a mixtnre of acrylic acid and an ester of acrylic acid, (B) styrene, (C) divinylbenzene, said divinylbenzene being present in an amount not more than 8.0 mol percent of the copolyrneric composition, and (D) an addition polymerization Catalyst.

14. A homogeneous solid resinous permselective membrane according to claim 10 wherein the polymerization product is a mixture consisting essentially of (A) a mixture of an ester of acrylic acid and maleic anhydride (B) styrene, (C) divinylbenzene, said divinylbenzene being present in an amount not more than 8.0 mol percent of the copolymeric composition, and (D) an addi tion polymerization Catalyst.

15. A homogeneous solid resnous permselective membrane according to claim 10 wherein (I) is benzoyl peroxde, (IIA) maleic anhydride and (IIB) styrene.

16. A permselective membrane formed in accordance with the process of claim 4 wherein the ion-exchangeactive material is present in an amount between about ten percent and about seventy-five percent of the polymerizable material, and containing at least twenty percent water of hydration.

17. A permselective membrane formed in accordance with the process of claim 4 wherein the ion-exchangeactive material is present in an amount between about fteen and about fifty percent, and containing at least twenty percent water of hydration.

18. The process for the production of a homogeneous solid resinous cationic permselective membrane which 16 consists essentially of polymerizing a mixture consisting essentially of (I) an addition polymerization catalyst and (II) a mixture of monomers `capable of addition copolymerization consisting essentially of (A) an ionexchange-active material selected from the group consisting of (1) a polymerizable olefinc monomer containing a carboxylic group (2) a rnixture of a polymerizable olefinic monomer containing a carboxylic ester group and a polymerizable olenic monomer containing a carboxyl group, (3) a mixture of a polymerizable olefinic monomer containing a carboxylic amide group and a polymerizable olefinic monomer containing a carboxyl group, (4) a mixture of a polyrnerizable olefinic monomer containing a nitrile group and a polymerizable olefinic monomer containing a carboxyl group, and (5) mixtures thereof, (B) a polymerizable ion-exchangeinactive olefinic monomer and (C) a cross-linking olefinic monomer containing at least two polymerizahle carbon-to-carbon double bonds, said cross-linking olefinic monomer being present in an amount not more than 8.0 mol percent of the dry copolymeric composition, and said ion-exchange-inactive monomer and crosslinking olefinic monomer being present in said membrane in an amount between about twenty-five mol percent and about ninety mol percent of the dry polymerized materials; producing a dry, homogeneous, polymerized membrane; and subsequently converting the potentially active functional groups to carboxyl groups and introducing water of hydration into the membrane by hydrolyzing in an aqueous solution containing a hydrolysis Catalyst to the extent necessary to produce carboxyl groups uniformly distributed throughout said membrane in 'an amount from about one to about six milliequivalents per gram of dry membrane.

19. The permselective membrane formed in accordance with the process of claim 18.

` 20. A cell for the electrolytic decomposition of an aJkali metal halide which consists essentially of a container, an anode, a cathode, which contains interposed between said anode and said cathode a permselective membrane made in accordance with the process of claim 1.

References Cited in the file of this patent UNITED STATES PATENTS 2,439,227 Seymour et al Apr. 6, 1948 2,675,370 Barrett Apr. 13, 1954 2,719,136 Caldwell Sept. 27, 1955 2,730,768 Clarke Jan. 17, 1956 2,731,408 Clarke Ian. 17, 1956 2,891,015 Tsunoda June 16, 1959 

1. A PROCESS FOR THE PRODUCTION OF SOLID, RESINOUS, PERMSELECTIVE MEMBRANE WHICH CONSISTS ESSENTIALLY OF POLYMERIZING A MIXTURE CONSISTING ESSENTIALLY OF (I) AN ADDITION POLYMERIZATION CATALYST, AND (II) A MIXTURE OF MONOMERS CAPABLE OF ADDITION COPOLYMERIZATION CONSISTING ESSENTIALLY OF (A) AN ION-EXCHANGE-ACTIVE MATERIAL SELECTED FROM THE GROUP CONSISTING OF (1) A POLYMERIZABLE OLEFINIC MOMOMER CONTAINING A CARBOXYLIC GROUP, (2) A POLYMERIZABLE OLEFINIC MONOMER CONTAINING A GROUP WHICH MAY BE SUBSEQUENTLY HYDROLYZED TO FORM A CARBOXYLIC GROUP, (3) MIXTURES OF (1) AND (2), AND (4) A POLYMERIZABLE OLEFINIC MONOMER CONTAINING AN ANIONEXCHANGE GROUP, AND (B) A POLYMERIZABLE ION-EXCHANGEINACTIVE MONOMERIC DILUENT SELECTED FROM THE GROUP CONSISTING OF (A) A POLYMERIZABLE OLEFINIC MONOMER, (B) A CROSS-LINKING OLEFINIC MONOMER CONTAINING AT LEAST TWO POLYMERIZABLE CARBON-TO-CARBON DOUBLE BONDS IN AN AMOUNT NOT MORE THAN 8.0 MOL PERCENT OF THE COPOLYMERIC COMPOSITION, AND (C) MIXTURES OF (A) AND (B), PRODUCING A DRY HOMOGENEOUS POLYMERIZED MEMBRANE, AND SUBSEQUENTLY CONVERTING THE POTENTIALLY ACTIVE FUNCTIONAL GROUPS TO ION-EXCHANGE GROUPS AND INTRODUCING WATER OF HYDRATION INTO THE MEMBRANE BY HYDROLYZING IN AN AQUEOUS SOLUTION CONTAINING A HYDROLYSIS CATALYST TO THE EXTENT NECESSARY TO PRODUCE ION-EXCHANGE GROUPS UNIFORMLY DISTRIBUTED THROUGHOUT SAID MEMBRANE IN AN AMOUNT FROM ABOUT ONE TO ABOUT SIX MILLIEQUIVALENTS PER GRAM OF DRY MEMBRANE. 